Combined emissivity and radiance measurement for the determination of the temperature of a radiant object

ABSTRACT

A system and method of measurement of emissivity and radiance of a wafer in a rapid thermal processing chamber enables determination of wafer temperature and control of temperature of the wafer. Mirrors enclose the chamber and reflect radiation from lamps within the chamber to heat the workpiece of interest. One or more viewing ports are provided in one of the mirrors to allow for the egress of radiant energy emitted by the wafer. The wavelength of the exiting radiation is selected by an optical filter having a passband which passes radiation at wavelengths emitted by the wafer while excluding radiation emitted by heating lamps. A chopper having surface regions differing in their reflectivity and transmissivity is positioned along an optical path of radiation propagating through the one or more ports, this resulting in a pulsation of detected radiation. The ratio of the detected intensities of the radiation pulses is used to determine wafer reflectance based on reflectivity and transmissivity of the reflective portion of the chopper. The maximum intensity of radiation is also taken as a measure of radiance. The reflectance is employed to calculate the emissivity, and the emissivity in combination with the radiance are employed to calculate the wafer temperature.

BACKGROUND OF THE INVENTION

This invention relates to temperature measurement of an object in aradiative environment, such as a semiconductor wafer in a rapid thermalprocessing (RTP) chamber, by computation of the temperature fromnoncontact in-situ measurement of emissivity and radiance of the object.As a particularly advantageous feature, the emissivity is determined insitu and in real-time by measuring wafer reflectance without an externallight source through the use of a chopper having regions of differenttransmissivity and reflectance. The ratio of light intensities detectedthrough the open and reflecting portions of the shutter is used todetermine the emissivity of the wafer for pyrometric temperaturemeasurement purposes. Furthermore, in situ, real-time radiancemeasurements are obtained through the open region of the shutter.Interference due to radiation from the heating lamps is excluded byappropriate wavelength selection and by the use of a light pipe thatcollects light from a defined direction. This technique enablessingle-point measurements of the temperature of a hot object in aradiative environment, and it can be used for precise control oftemperature in this environment. Through the use of several sensors atseparate locations across the wafer along with appropriate control ofthe lamp intensities in different zones of the reactor, it is possibleto control the temperature profile (uniformity) across the wafer duringthe initial heating ramp as well as at steady state. Since thetemperature measurement is attained in real time, it can be usedadvantageously to control both the heating of the chamber and thetemperature of the heated object.

In manufacturing processes, the heating of a workpiece is employed oftenin one or more of the manufacturing steps in order to obtain a desiredend product. This is particularly true in the case of manufacture ofsemiconductor circuits and other semiconductor products wherein a waferis treated by various steps of etching, doping and cleaning to constructa complex configuration of circuit components on the wafer. Duringvarious stages of the manufacture, the temperature of the wafer must beelevated, for example, to temperatures in the range of 600-1200 degreesCentigrade. It is the practice to place the wafers in a furnace toobtain the desired temperature; however, more recently the use of RTPapparatus has come into use as a more attractive alternative to furnaceprocessing of wafers because RTP provides for a rapid, single waferprocessing with advantages of better control and lower manufacturingcosts. However, especially in view of the short duration of many RTPsteps, it becomes increasingly important to measure and to control thetemperature closely. Control of temperature requires a measurement ofthe temperature, and it is advantageous to employ in-situ temperaturemeasurement by a viewing of radiation of the wafer, or other workpiece,without physically contacting the wafer. Such a temperature measurementmay be done at several discrete points on a wafer to obtain data of anynonuniformity in temperature, or temperature profile, across the wafer.

Accurate measurement of temperature is obtained from a combination ofboth emissivity data and radiance data of a workpiece or other object ina radiative environment wherein the data is obtained at a specificwavelength, and the temperature is calculated from this data. Animprovement in the measurement of emissivity directly benefits thetemperature measurement and allows for a more accurate determination ofthe temperature.

Typically, emissivity of an object has been determined from a measure ofreflectance of the object. In the past, such measurement has employeddirectional reflectance using a filtered lamp radiation or a laser as asource of light at an appropriate wavelength. Such techniques are taughtin the following U.S Pat. Nos: 4,647,774 (Quantum Logic Corp., Mar. 3,1987), 4,956,538 (Texas Instruments, Inc., Sep. 11, 1990), 5,156,461(Texas Instruments, Inc., Oct. 20, 1992), and 4,919,542 (A.G. ProcessingTechnology, Apr. 24, 1990). The light, typically infrared light, isdirected as an incident beam at the object from a point external to theradiative environment of the object. Light from the incident beam isreflected from the object as a reflected beam. In a situation of majorconcern herein, the object is a semiconductor wafer, and the radiativeenvironment is an RTP chamber enclosing the wafer. The intensities ofthe incident and the reflected beams in a given direction are used toderive the wafer reflectivity at the desired wavelength. The fraction ofthe reflected light collected in the chosen direction is also measuredusing scattering measurements described in U.S. Pat. No. 5,156,461(Texas Instruments, Inc.) The measured bidirectional reflectance incombination with the specularity measurements, yield information aboutthe total reflectivity of the wafer. A disadvantage of this method isthe need for additional hardware in the form of an external light sourcewhich increases the cost and size of the temperature sensing system.

Other apparatus employed in the measurement of emissivity includes theuse of two fibers mounted in an RTP chamber in such a fashion that oneof the fibers views the output of lamps used to heat the wafer, and asecond of the fibers views radiation coming from the wafer, the latterincluding thermal wafer emissions as well as the lamp radiationreflected from the wafer. This technique is described in U.S. Pat. No.5,154,512 (Luxtron Corp.). Typically, the measurements are made at0.9-1.5 micron wavelength where the lamp intensity is high and the waferis opaque. This method makes use of the fact that the intensity of thelamp emission, as well as the reflected lamp radiation, has an ACcomponent due to imperfect electrical filtering in the supplying ofcurrent to the lamp, while the wafer thermal emission is continuousbecause of its long thermal time constant. The reflectance is thenmeasured as the ratio of the AC components of the light from the twofibers. Emissivity is then calculated from Kirchoff's Law. In thismethod, since the light is collected from a fairly large region on thewafer, the signal of the reflected radiation is not extremely sensitiveto surface roughness. A disadvantage of this method includes the need tomodify the housing of the RTP apparatus to allow the sensors to beinstalled at appropriate locations.

There are several well-known and severe problems when emissivity is notmeasured during the RTP process due to a variation in the emissivity ofthe object. Such variation degrades the accuracy and repeatability ofpyrometric temperature measurements by introducing errors as much asseveral tens of degrees Centigrade. In general, the wafer emissivity isnot constant from wafer to wafer and for a given wafer during amanufacturing process because the emissivity is a function of a numberof variables including wafer temperature, surface film thicknesses,films on the heating chamber wall which alter the chamber reflectivity,backside roughness of the wafer, and process history. Therefore, it isnecessary to monitor the emissivity of wafers in situ and in real time,and in a non-contact manner to insure correct temperature measurement.The use of other forms of measurements wherein the object must beremoved from its environment, or by use of monitor wafers to provide asubstitutional form of measurement, or the use of theoreticalcalculations to estimate emissivity do not provide the desired amount ofaccuracy because of variations in the manufacturing process and in thewafer itself. It is known that, by use of Kirchoff's Law, emissivity canbe determined solely from reflectance measurements for the case whereinthe wafer is opaque, including the situation wherein the opacity isinduced by temperature or by doping. For highly doped wafers, or formeasurement of wavelength less than approximately one micron, emissivitymeasurement from reflectance alone is possible for objects at roomtemperature and above. However, for emissivity measurements conducted atlonger wavelengths or for lower doping, such as at a wavelength of threemicrons for an undoped silicon wafer, and in the absence of films on thewafer, determination of emissivity from reflectance measurements alonecan be accomplished only at temperatures higher than approximately 700degrees Centigrade wherein the wafer is opaque.

A problem arises in that presently available apparatus and proceduresare not readily implemented for standard forms of RTP apparatus withoutextensive modification and/or introduction of ancillary hardware to theRTP apparatus, this creating a need for measurement equipment andprocedure which are easier to implement.

SUMMARY OF THE INVENTION

The aforementioned problem is overcome and other advantages are providedby a system and methodology of temperature measurement which, inaccordance with the invention, employ measurement of reflectance of anobject or workpiece within a radiative environment such as an RTPchamber, for determination of emissivity of the object from areflectance measurement without the use of an external light source. Thespectral region where the emissivity can be measured is between 1 and 5μm. In this wavelength range, light is emitted by both the hot wafer andthe heating lamps. However, at wavelengths longer than 4.5 μm, radiationemitted by the heating lamps is significantly attenuated by the quartzenvelope which surrounds the lamps. Furthermore, there is a calculationof the temperature of the workpiece from emissivity and radiance data ofthe workpiece at a specific wavelength of the measurement.

The invention enables an accurate measurement of the object temperatureto be attained, and this temperature measurement may be applied to acontroller of the lamp power supply to adjust heating of the object foraccurate maintenance of a desired temperature. It is noted that theforegoing system and methodology accomplishes the foregoing measurementwithout providing for any physical contact of the measuring instrumentswith the object, and, furthermore, allows the measurement to take placewhile the object remains in its place within the chamber. Furthermore,the measurement is obtained by a direct viewing of the object itself soas to obviate the need for use of monitor wafers to obtain an estimateof the measurement.

The invention is operative with various configurations of standard,commercially available RTP chambers, such as a chamber having a singlecentrally located viewing port and a chamber having a pair of viewingports located near opposite ends of the chamber. Measurements ofreflectance and radiance are conducted by use of the existing viewingports without requiring modification of the RTP chamber. One preferredembodiment of the invention is operative with the single central viewingport, and a second embodiment of the invention is operative with thepair of spaced apart viewing ports. Each embodiment of the inventionemploys a detector of radiation, the radiation propagating via a viewingport from the object, and an optical shutter or chopper. The chopper hasregions differing in transmissivity and reflectivity, and is employedfor modulating the intensity of rays of radiation propagating betweenthe object and the detector in order to determine the reflectivity ofthe object. Both the chopper and the detector are located outside of theRTP chamber. By way of example in describing the invention, the objectis presumed to be a semiconductor wafer.

With respect to the first embodiment of the invention, the RTP apparatushas a plurality of heating lamps or elements for heating a wafer withinthe chamber, and includes opposed mirrors for reflecting lamp radiationor light upon the wafer. The single viewing port is provided in acentral region of a first one of the mirrors. Typically the foregoingwafer surface is flat. The central location of the port permits thedetector to view the wafer along a line of sight which is perpendicularto the surface of the wafer. A chopper, positioned between the port andthe detector, has a transparent or open region, and a reflective regionwhich is partially transmissive.

Upon rotation of the chopper disk, the transparent and reflectiveportions pass in alternating fashion in front of the port to modulatethe intensity of the light incident upon the detector. The modulationproduces an alternating succession of strong and weak pulses of light.If desired, a sapphire rod or other light guide may be passed throughthe port to control the direction of the light collection. The sapphirerod collects light from a solid angle determined by its numericalaperture and refractive index. It is compatible with elevatedtemperature operation, and it is transparent in the wavelength region ofinterest. Appropriate positioning of the quartz rod enables lightcollection from the wafer and not from other regions of the quartzchamber. The accuracy of this temperature measurement technique can befurther enhanced by the selection of the appropriate spectral wavelengthto avoid interference from radiation emitted from the heating lamps. Theratio of intensity of the weak pulses to the intensity of the strongpulses is dependent, by a mathematical relationship, on the reflectanceof the wafer as well as on the reflectance and transmittance of anyreflecting surfaces in the optical path outside of the chamber, such asthe transmittance and reflectance of the regions of the chopper. Thelatter two parameters are known, and the intensity ratio is measured topermit a solving of the mathematical relationship for the waferreflectance.

In the second embodiment of the invention, the lamps and a reflector arelocated on one side of the chamber for illuminating a top side of thewafer, and the two viewing ports are located in a housing of the RTPapparatus facing the bottom side of the wafer. The spaced-apartlocations of the two viewing ports permit a viewing of the wafer, viaeither one of the ports, by the detector along a line of sight which isangled relative to the bottom wafer surface. The line of sightintercepts the bottom wafer surface at a viewing site. The detectorviews the wafer via a fixed partially transmissive reflector. Thechopper has a transparent or open region and a fully reflective region.The chopper is positioned along a second line of sight passing throughthe second viewing port to the viewing site on the wafer. The two linesof sight are angled equally to the wafer surface and provide a singleoptical path wherein light propagating from the reflective region of thechopper toward the wafer is reflected at the wafer viewing site to thedetector. Operation of the chopper provides for modulation of theintensity of light propagating along the optical path to the detector.In a manner similar to the foregoing operation of the first embodiment,in the operation of the second embodiment, there is a mathematicalrelationship between the intensity ratio of the light pulses and of thereflectance and transmittance of optical elements of the optical path,including the reflectance of the wafer, which can be solved for thewafer reflectance.

A third embodiment of the invention is similar to the second embodimentand provides for emplacement of the chopper in front of the detector inplace of the partially transmissive reflector. A fixed fully reflectivemirror is located on the second line of sight in place of the chopper.In the operation of the third embodiment, there is also a mathematicalrelationship between the intensity ratio of the light pulses and of thereflectance and transmittance of optical elements of the optical path,including the reflectance of the wafer, which can be solved for thewafer reflectance.

In the three embodiments of the invention, the light pulses obtained bythe open region of the chopper, and sensed by the detector, may be takenas a measure of the radiance of the wafer. The emissivity is calculatedfrom the reflectance, and the temperature is calculated from theemissivity and the radiance at a specific value of wavelength of thelight. The temperature may be employed by a controller of a power supplywhich energizes the lamps, thereby to provide a feedback loop formaintaining a desired temperature of the wafer in the radiativeenvironment.

It is noted that the invention is particularly suited for measurement oftemperature of a wafer having a flat surface for directing rays ofradiation to a detector for the radiance and reflectance measurements.However, in the general situation, the invention may be employed formeasuring temperature of some other object having a different shape, andpossibly within some other radiative environment such a heating chamberoperative with a concave mirror for reflective feedback of light uponthe object.

BRIEF DESCRIPTION OF THE DRAWING

The aforementioned aspects and other features of the invention areexplained in the following description, taken in connection with theaccompanying drawing figures wherein:

FIG. 1 is a diagrammatic view of an RTP apparatus and an optoelectronicsystem for analyzing radiation emitted via a port to measure emissivityand radiance of a workpiece within the chamber, the system providing fora determination of temperature of the workpiece from emissivity andradiance data;

FIG. 2 is a modification of view of FIG. 1 showing a sapphire rod usedas a light conduit in the port;

FIG. 3 is a diagrammatic view of an alternative embodiment of theoptoelectronic system of FIG. 1 for use with an RTP apparatus ofdifferent configuration from the RTP apparatus of FIG. 1, wherein thesystem is operative with two spaced-apart ports in a housing of the RTPapparatus for viewing the workpiece;

FIG. 4 is a block diagram of a gated integrator of FIGS. 1 and 2; and

FIG. 5 is a diagrammatic view of a third embodiment of the inventionsimilar to that of FIG. 2.

Identically labeled elements appearing in different ones of the figuresrefer to the same element in the different figures.

DETAILED DESCRIPTION

FIG. 1 shows a system 10 for measurement of emissivity of asemiconductor wafer 12 in the radiative environment of a heatingapparatus 14, such as RTP apparatus, and for controlling the apparatus14 to maintain a desired temperature of the wafer 12. It is to beunderstood that the use of the RTP apparatus in the preferred embodimentof the invention is provided by way of example in the practice of theinvention, and that the theory of the invention applies to other formsof heating apparatus and to objects or workpieces other than thesemiconductor wafer 12. The heating apparatus 14 includes a chamber 16which encloses the wafer 12, and comprises a top wall 18, a bottom wall20, and sidewalls 22. The wafer 12 has a flat planar form, as do the topand bottom walls 18 and 20, and is held between the top and the bottomwalls 18 and 20 by supports 24 extending inwardly from the sidewalls 22.

Rows of heating lamps 26, external to the chamber 16 illuminate thewafer with radiant energy to heat the wafer 12. The lamp 26 may be anywell known form of heating lamp or section of a heating filament. Thewalls of the chamber 16 are fabricated of a material, such as quartz,which is transparent to the radiation of the lamps 26. An upper row ofthe lamps 26 is provided above the top wall 18, and a lower row of thelamps 26 is positioned below the bottom wall 20. The heating apparatus14 further comprises a top mirror 28 located above the upper row of thelamps 26 and a bottom mirror 30 located below the bottom row of thelamps 26 for reflecting rays of the lamp radiation onto the wafer 12.Electric current for energizing the lamps 26 is provided by a powersupply 32 operative in response to an electric signal from a controller34 for maintaining a desired temperature of the wafer 12. The reflectingsurfaces of the mirrors 28 and 30 are planar and parallel to each otherand to the wafer 12. The heating apparatus 14 is provided with a port 40for viewing the wafer 12, the port 40 being formed as an aperture withinthe bottom mirror 30 and being located approximately equidistant betweentwo neighboring lamps 26.

In accordance with the invention, the system 10 includes an opticalshutter, or chopper 42 located within an optical path 44 of radiationpropagating through the port 40 in a direction normal to the bottomsurface of the wafer 12. The chopper 42 comprises a disk 46 disposed ona shaft 48 connected to a driver 50 which imparts rotation to thechopper 42. The disk 46 has an opening 52 which serves as a fullytransmissive region of the chopper 42. The disk 46 supports a partiallytransmissive mirror 54. The film 56 on the substrate 56A constitutes areflecting surface and is disposed in a plane perpendicular to theoptical path 44.

The system 10 also includes a detector 58 of the radiation propagatingalong the path 44, there being a filter 60 located on the optical path44 and positioned directly in front of the detector 58. The filter 60has a passband centered at the frequency at which the emissivity and theradiance are to be measured. In one embodiment, the passband liesoutside the spectral transmission band of the thick quartz of the bodyof the chamber 16, at wavelengths greater than approximately 4.5 micron,to exclude radiation of the lamps 26 from the detector 58 duringmeasurement of the wafer radiance. Interference due to radiation fromthe heating lamps is further reduced by the use of a light guide such asthe sapphire rod discussed above. Upon rotation of the chopper 42, theintensity of light reaching the detector 58 via the path 44 is modulatedsuch that, upon passage of the light via the opening 52, full intensityof the light is received at the detector 58. Upon interposition of thepartially reflecting mirror 54 in the optical path 44, part of theradiant energy is reflected by the mirror 54 back into the chamber 16,and part of the radiant energy propagates through the mirror 54 and thefilter 60 to the detector 58. With respect to such part of the radiantenergy which is reflected by the mirror 54, it is noted that there aremultiple reflections of radiation between the lower reflecting surfaceof the wafer 12 and the mirror 54. In a manner to be describedhereinafter, the reflectivity of the wafer is derived from the ratio ofthe light intensities propagating through the two regions of thechopper. From the reflectivity, the emissivity of the wafer isdetermined. It is convenient to identify the opening 52 of the chopper42 as channel A, and the mirror 54 as channel B. The intensity ofradiation received at the detector 58 and propagating via channel B istypically less than the intensity of radiation propagating via channelA.

The system 10 further comprises a signal processor 62 such as a gatedintegrator or a waveform averager, an emissivity calculation unit 64,and a temperature calculation unit 66. In operation, the detector 58outputs an electric signal to the signal processor 62 representing theintensity of radiation detected by the detector 58. The signal processor62 is operative, in a manner to be described hereinafter, to output twosignals, one of which is the ratio of intensities of the signal ofchannel B to the signal of channel A, and the other output signal whichis the intensity of the signal of channel A. The ratio of theintensities of the signals of channel B and channel A are outputted vialine 68 to the emissivity calculation unit 64. The intensity of thesignal at channel A is outputted via line 70 to the temperaturecalculation unit 66.

The intensity ratio on line 68 is employed by the emissivity calculationunit 64, in a manner to be described hereinafter, to calculate thereflectance of the wafer 12, the reflectance then being used to give theemissivity of the wafer 12. The measured bidirectional reflectancecombined with the known specularity of the wafer yields the totalreflectance of the wafer for the purposes of determining its emissivity.The specularity would be measured prior to operation of the system 10and stored at 72 for use by the emissivity calculation unit 64. Theintensity of the signal of channel A, at line 70, serves as a measure ofthe radiance of the wafer 12.

The temperature calculation unit 66 employs the radiance of line 70 andthe emissivity outputted by the unit 64 to calculate the temperature ofthe wafer 12. A signal representing the temperature measurement isoutputted via line 74 from the temperature calculation unit 66 to thecontroller 34. The value of radiation wavelength to be employed in thecalculation of the temperature by the unit 66 is stored at 74 andinputted to the unit 66 for calculation of the temperature. Details inthe operation of the emissivity calculation unit 64 and the temperaturecalculation unit 66 will be provided hereinafter. The controller 34 isresponsive to the temperature signal on line 74, and to a referencetemperature input 76 for outputting a control signal to the power supply32. The control signal to the power supply 32 commands the requisiteamount of current to the lamps 26 for maintaining the wafer temperaturesubstantially equal to the reference temperature.

FIG. 2 shows a system 10A which is an alternative embodiment of thesystem 10 of FIG. 1. The system 10A has essentially the same componentsas the system 10, but further comprises a rod 78 inserted within theport 40 and oriented along the optical path 44. The rod 78 extends intothe heating apparatus 14 to a point beyond the lower row of lamps 26,and extends outwardly from the heating apparatus 14 to a locationimmediately in front of the chopper 42. The rod 78 is made of a materialwhich is transparent to the radiation emitted by the wafer 12. In apreferred embodiment of the invention, the rod 78 is made of sapphire.Appropriate positioning of the sapphire rod minimizes the collection oflight from the heating lamps and maximizes the optical signal due toemission from the wafer. Interference due to lamp radiation is furtherreduced by detecting emission in an optical region outside the rangewhere light is transmitted through the thick quartz walls of the chamberbut within the transmission region of a thinned region of the quartzprovided to view the wafer.

An additional aspect in the operation of the systems of both FIGS. 1 and2 is the fact that the partially reflecting mirror 54 of the chopper 42,in cooperation with reflectance of the wafer 12, provides for asuccession of reflections of radiant energy along the path 44. In amanner to be described hereinafter, the reflectivity of the wafer isdetermined from the ratio of the light intensities propagating throughthe two regions of the chopper. From its reflectivity, the emissivity ofthe wafer is determined directly.

FIG. 3 shows a system 10B which is a further embodiment of theinvention, the system 10B employing essentially the same components asthe system 10 of FIG. 1, but employing a heating apparatus 14A having aconfiguration which differs from the configuration of the heatingapparatus 14 of FIGS. 1 and 2. In FIG. 3, the heating apparatus 14Acomprises a housing 16A fabricated of metal walls which enhancereflection of radiant energy within the housing. The housing 14A isdivided by a transparent wall in the form of a quartz plate 18A into anupper section and a lower section. The wafer 12 is located in the lowersection, and a plurality of the heating lamps 26 is located in the uppersection. Radiation from the lamps 26 propagates through the quartz plate18A to heat the wafer 12. Three mirrors 28A, 28B, and 28C are positionedbehind the lamps 26 for reflecting light of the lamps 26 toward thewafer 12. Two viewing ports 82 and 84 which are spaced apart from eachother are provided for viewing the wafer 12 at a point P from twodifferent directions along optical paths 86 and 88. The ports 82 and 84are angled relative to the bottom surface of the wafer 12, the latterbeing parallel to the plate 18A, and are located in a bottom wall 90 ofthe housing 16A. The ports 82 and 84 have a tubular shape and are closedoff by quartz windows 82A and 82B. Also provided in the housing 16A, andpartially shown in phantom view, is a central support 24A upstandingfrom the bottom wall 90 for holding the wafer 12. The support 24A may bemade of quartz, as is well known, and has a configuration permitting theviewing of the wafer 12 along the paths 86 and 88.

The system 10B further comprises a chopper 42A disposed on the opticalpath 86 and intercepting the optical path 86. The chopper 42A has atransparent region in the form of the opening 52 as does the chopper 42of FIGS. 1 and 2, but differs from the chopper 42 of FIGS. 1 and 2 inthat, in FIG. 3, the chopper 42A has a fully reflective mirror 92 whichis normal to the optical path 86. Also included in the system 10B is apartially reflecting mirror 94 which is positioned on the optical path88 with an orientation which is normal to the path 88. The mirror 94 ispositioned in front of the filter 60 which, in turn, is positioned infront of the detector 58. The mirror 94 allows for a portion ofradiation propagating along the path 88 to be transmitted via the filter60 to the detector 58, and for a portion of the radiation propagatingalong the path 88 to be reflected back to the point P.

In operation, upon rotation of the chopper 42A by the driver 50, theopening 52 and the mirror 92 are brought alternately into a position ofintercepting the path 86. Upon emplacement of the opening 52 in theoptical path 86, all of the radiation propagating along the path 86passes through the opening 52 and is lost. Upon emplacement of themirror 92 within the optical path 86, all of the radiation is reflectedby the mirror 92 back along the path 86 to the point P. The lowersurface of the wafer 12 reflects light incident along the path 86 topropagate out of the housing 16A along the path 88 and, similarly, lightincident upon the point P via the path 88 is reflected by the wafer 12out of the housing 16A along the path 86.

The foregoing discussion of the increased radiation intensity along thepath 44 in the systems of FIGS. 1 and 2 by multiple reflections betweenthe chopper mirror 54 and the wafer 12 applies also, in analogousfashion, to the system of FIG. 3 wherein, upon the presence of thechopper mirror 92 in the optical path 86, there is a succession ofreflections of radiant energy back and forth between the chopper mirror92 and the mirror 94. As a result, the intensity of the radiationincident on the path 88 towards the detector 58 is higher during thepresence of the chopper mirror 92 in the optical path 86 than during thepresence of the chopper opening 52 in the optical path 86.

In FIG. 3, during rotation of the chopper 42A, the intensity of theradiation on the path 88 varies in repetitive fashion between higher andlower values of intensity. These values of intensity are outputted bythe detector 58 along line 96 to the signal processor 62 in the mannerof a train of electrical pulses varying in amplitude periodically insynchronism with the rotation of the chopper 42A. The filter 60 operatesin the same fashion for filtering the radiation incident upon thedetector 58 as has been disclosed previously for FIG. 1. Thus, thesignal outputted by the detector 58 on line 96 in FIG. 3 is, apart froma scale factor, the same as that outputted by the detector 58 on line 96in FIG. 1. Also, in each of the embodiments of the system 10, 10A and10B of FIGS. 1-3, the driver 50 outputs a trigger signal on line 98which triggers the signal processor 62 to operate in synchronism withthe rotations of the chopper. By analogy with the operation of thesystem 10 of FIG. 1, in the system 10B of FIG. 3, channel A designatesthe signal for the open portion of the chopper and channel B designatesthe signal for the mirrored portion of the chopper.

In one embodiment of the invention, the signal processor 62 shown inFIG. 4 is commercially available gated integrator and, accordingly, onlya brief description of its circuit is provided herein. The integrator 62comprises a switch 100, two box-car detectors 102 and 104, an arithmeticlogic unit 106, a timing unit 108, a comparator 110, a reference signalsource 112 providing a reference signal to an input terminal of thecomparator 110, and a logic unit 114 determining whether an input signalon line 96 is from channel A or channel B. In operation, the triggersignal on line 98 synchronizes the timing unit 108 with rotation of thechopper 42, 42A, so as to provide timing signals for driving the switch100 and the box-car detectors 102 and 104. The timing unit 108 alsoprovides timing signals for operation of the arithmetic logic unit 106and the logic unit 114. The switch 100 transmits an input signal on line96 to the box-car detector 102 during one of the channel positions ofthe chopper, and to the box-car detector 104 during the other of thechannel positions of the chopper. Each of the detectors 102 and 104 isoperative to integrate and to store the signal on line 96 for theduration of each channel position of the chopper. The signal on line 96is applied also to the comparator 110 which determines whether thesignal is greater than or less than the reference signal provided by thesource 112. The output of the comparator 112 is applied to the logicunit 114 which determines that channel A is present for a high outputsignal of the comparator 110 and that channel B is present for a lowoutput signal of the comparator 110. The channel identificationoutputted by the logic unit 114 is applied to the arithmetic logic unitto enable it to perform arithmetic operations on the signals outputtedby the detectors 102 and 104. The arithmetic logic unit 106 then outputsthe ratio of the channel B signal to the channel A signal on line 68 andoutputs the channel A signal on line 70.

In FIG. 5, there is shown a system 10C which is substantially the sameas the system 10B of FIG. 3 except that, in FIG. 5, the location of thechopper has been changed. The chopper 42 is employed on the optical path88, in place of the mirror 94, and a fixed fully reflective mirror 116is disposed on the optical path 86. Radiation is reflected back andforth along the paths 86 and 88 between the fixed mirror 116 and thechopper mirror 54 via the surface of the wafer 12. Alternate interposingof the chopper mirror 54 and the chopper opening 52 in the path 88modulates the amplitude of the radiation intensity detected by thedetector 58 to provide for a detector output signal on line 96 havingessentially the same pulsed waveform as the signals on line 96 in theembodiments of FIGS. 1-3.

The emissivity calculation unit 64 is operative to calculate thereflectance of the wafer 12, for the embodiments of the inventiondepicted in FIGS. 1 and 2 by the following mathematical description:

R_(W) =wafer reflectance

R_(R) =reflectance of enclosure

R_(M) =reflectance of external mirror

T_(M) =transmittance of external mirror

I_(L) =radiance of extended lamp source at λ_(o) ±Δλ

I_(W) =radiance of wafer at λ_(o) ±Δλ

No mirror in path (channel A):

Signal (channel A)=I_(W) +I_(L) R_(W) +I_(L) R_(R) R_(W) +I_(L)

Mirror in path (channel B): ##EQU1##

The mathematical description is given in terms of the wafer reflectanceR_(W), the reflectance of the enclosure represented by the bottom mirror30 and given by R_(R), the reflectance of the external mirror providedby the reflecting surface 56 and given by R_(M), the transmittance ofthe external mirror given by T_(M), the radiance of the source of lightrepresented by the lamps 26 and given by I_(L), and the radiance of thewafer 12 given by I_(W).

The detected signal for channel A (on line 96) is given by

    I.sub.W +I.sub.L R.sub.W +I.sub.L R.sub.R R.sub.W +I.sub.L

The detected signal for channel B (on line 96) is given ##EQU2## whichis equal to ##EQU3## which is equal to ##EQU4##

The ratio of the two channel signals is given by ##EQU5##

With respect to the foregoing equation; the ratio of the channel signals(B/A) is outputted on line 68 from the gated integrator 62 and, hence,is known. The quantities R_(M) and T_(M) are also known. Therefore, theforegoing equation can be solved for the directional reflectance of thewafer.

The emissivity calculation unit 64 is operative to calculate thereflectance of the wafer 12, for the embodiments of the inventiondepicted in FIG. 3 by the following mathematical description:

R_(M1) =reflectance of mirror M1 on chopper

R_(W) =wafer reflectance

T_(M2) =transmittance of mirror M2

R_(M2) =reflectance of output coupling mirror

I_(L) =radiance due to lamp light in chamber

I_(W) =radiance from wafer

Signal (channel A) with mirror M1 not in path=I_(W) +I_(L) +I_(L) R_(W)##EQU6## which is solved for RW, the wafer reflectance.

A corresponding mathematical explanation of the operation of theembodiment of FIG. 5 may be derived in a manner similar to the foregoingmathematical explanations of the embodiments of FIGS. 1-3 and,accordingly, need not be presented herein.

The emissivity calculation unit 64 is operative to provide theemissivity for all of the embodiments of FIGS. 1-3 by solving thefollowing equation;

    ε=1-R

Where ε is emissivity, and R is total reflectance of the wafer.

The temperature calculation unit 30 is operative to provide thetemperature, for all of the embodiments of the invention of FIGS. 1-3,by solving Plank's equation. ##EQU7## where R.sub.λ is radiance

T is temperature

λ is wavelength

ε.sub.λ is emissivity

c₁ and c₂ are constants

Thereby the invention has accomplished a major objective of providingfor an in-situ non-contacting method of measuring radiation emitted by awafer or other object within a radiative environment by a procedure ofmodulating the emitted radiation without need for an external source ofcoherent radiation, such as the light of an infrared laser, toaccomplish measurements of reflectance, emissivity, radiance andtemperature.

It is to be understood that the above described embodiments of theinvention are illustrative only, and that modifications thereof mayoccur to those skilled in the art. Accordingly, this invention is not tobe regarded as limited to the embodiments disclosed herein, but is to belimited only as defined by the appended claims.

What is claimed is:
 1. A system for measuring emissivity and radiance ofa radiant object, comprising:a detector of radiant energy emitted bysaid object, said detector being positioned for receipt of radiantenergy propagating along an optical path substantially normal to asurface of said object; a shutter assembly comprising a first regionwhich is transparent to the radiant energy, and a second region which ispartially reflective and partially transmissive to the radiant energy,said shutter assembly being operative to interpose alternately saidfirst region and said second region in said optical path at a locationbetween said object and said detector, said detector outputting a firstsignal upon interception of said optical path by said first region and asecond signal upon interception of said optical path by said secondregion, said first signal being stronger than said second signal; meansfor dividing the second signal by the first signal to provide areflectance of said object, the first signal serving as a measure ofradiance of said object; and means for deriving the emissivity of saidobject from the reflectance.
 2. A system for measuring emissivity andradiance of a radiant object for determination of the temperature of theradiant object, comprising:a detector of radiant energy emitted by saidobject, said detector being positioned for receipt of radiant energypropagating along an optical path substantially normal to a surface ofsaid object; a shutter assembly comprising a first region which istransparent to the radiant energy, and a second region which ispartially reflective and partially transmissive to the radiant energy,said shutter assembly being operative to interpose alternately saidfirst region and said second region in said optical path at a locationbetween said object and said detector, said detector outputting a firstsignal upon interception of said optical path by said first region and asecond signal upon interception of said optical path by said secondregion, said first signal being stronger than said second signal; meansfor dividing the second signal by the first signal to provide areflectance of said object, the first signal serving as a measure ofradiance of said object; means for deriving the emissivity of saidobject from the reflectance; and means responsive to said emissivity,said reflectance and said radiance to determine the temperature of saidobject.
 3. A system according to claim 2 wherein said system isoperative with a heating assembly including a heating chamber forreceiving said object, said object being a semiconductor wafer, saidheating chamber having a wall transparent to radiation at a designatedwavelength, said heating assembly comprising a row of heating elementsoutside said chamber and a mirror for reflecting radiant energy emittedby said heating elements to said wafer, said heating assembly having anaperture disposed on said optical path for allowing propagation ofradiant energy along said path from said wafer to said detector, saidpath being located between two of said heating elements; andwherein apartially reflective surface of said second region of said shutterassembly is oriented relative to said optical path for directingradiation toward said wafer to induce multiple reflections of radiantenergy between said second region and said wafer for increased signalstrength to said second signal.
 4. A system according to claim 3 whereinsaid shutter assembly is located outside of said heating assembly.
 5. Amethod for measuring emissivity and radiance of a radiant object,comprising steps of:positioning a detector for detecting radiant energyemitted by said object and propagating along an optical pathsubstantially normal to a surface of said object; placing alternately afirst shutter region and a second shutter region in said optical path ata location between said object and said detector, said first regionbeing transparent to the radiant energy, and said second region beingpartially reflective and partially transmissive to the radiant energy;outputting by means of said detector a first signal upon interception ofsaid optical path by said first region and a second signal uponinterception of said optical path by said second region, said firstsignal being stronger than said second signal; and dividing the secondsignal by the first signal to provide a reflectance of said object, thefirst signal serving as a measure of radiance of said object; andderiving the emissivity of said object from the reflectance.
 6. A methodfor measuring emissivity and radiance of a radiant object fordetermining the temperature of the radiant object, comprising stepsof:positioning a detector for detecting radiant energy emitted by saidobject and propagating along an optical path substantially normal to asurface of said object; placing alternately a first shutter region and asecond shutter region in said optical path at a location between saidobject and said detector, said first region being transparent to theradiant energy, and said second region being partially reflective andpartially transmissive to the radiant energy; outputting by means ofsaid detector a first signal upon interception of said optical path bysaid first region and a second signal upon interception of said opticalpath by said second region, said first signal being stronger than saidsecond signal; dividing the second signal by the first signal to providea reflectance of said object, the first signal serving as a measure ofradiance of said object; deriving the emissivity of said object from thereflectance; and determining the temperature of said object based onsaid emissivity, said reflectance and said radiance.